Mechanical filter

ABSTRACT

A drill string assembly containing a section of tube with a first end. The first end directly, mechanically links a drill pipe to a drill bit and transmits rotational forces from the drill pipe to the drill bit during drilling. A spring is connected to the section of tube at a point remote from the first end of the section of tube. A mass is connected to the spring such that the mass can vibrate on the spring in an axial direction relative to the section of tube and drill string. The value of the mass and the stiffness of the spring being selected such as to cause them to act as a mechanical filter and damp longitudinal compression waves at a predetermined frequency of frequency band and received at the first end of the section of tube.

BACKGROUND OF THE INVENTION

The present invention relates to a drill string sub assembly comprisinga mechanical filter.

Logging while drilling (LWD) apparatus is disclosed in U.S. Pat. No.5,387,767 which enables acoustic signals to be transmitted to rock abouta borehole, while minimising the energy transmitted into the drillcollar. However, when drilling a borehole it may be desirable to knowthe stratum which is about to be drilled through, that stratumimmediately ahead of the drill bit. This allows appropriate drillingparameters to be employed such as drill speed or the weight on the bit.This is particularly important when drilling for oil, for it enablesover pressure regions in the stratum ahead of the drill bit to beidentified and the fluid pressure within the borehole adjusted to aminimum safe level accordingly.

UK patent application, publication number 2288020 discloses a drill bitarrangement embodying an acoustic transducer which enables acousticsignals to be transmitted and received through the drill bit itself.This enables the region ahead of the drill bit to be surveyed withoutthe very expensive requirement of withdrawing the drill string andlowering a wire-line tool down the borehole including an acoustic sensorto survey the bottom of the borehole. The apparatus described in UKpatent application publication number 2288020 discloses that it isnecessary to isolate the drill bit and acoustic transducer from thedrill string in order to prevent excessive losses in transmitted andreceived signals from and to the transducer, caused by transmissionalong the drill string, and also to avoid problems caused by signalsbeing returned along the drill string or reflected by the drill string.

This earlier application discloses two embodiments for isolating thedrill bit and acoustic sensor from the drill string, and furtherdevelopment work on these resulted in the design illustrated in FIG. 1of the attached drawings, the essential elements of which are brieflysummarised below.

Referring to FIG. 1, a drill bit 10 is connected to a first hollowmember 11 which slidably extends within second hollow member 12. Twosets of rubber coated splines 13 on the two respective hollow membersengage and act to centralize the two members in the absence of anytorque applied to the drill string, while two sets of steel splines 14associated with each respective hollow member engage when high torqueloads are applied during drilling to transmit torque from the drillstring to the drill bit. A series of steel balls 15 engage with recesses16 in the first hollow member and act to limit the relative separationof the two hollow members.

During a drilling operation the downward force exerted by the drillstring causes the faces 17 of the two hollow members to come intocontact such that the weight on the drill string is applied directly tothe drill bit. When it is desired to survey the area of rock ahead ofthe drill string the weight on the drill string is reduced such that thefirst and second hollow members extend and adopt the positionillustrated in FIG. 2. Compliances 18 acting between the first andsecond hollow members act to maintain a desired pressure on the drillbit to ensure good acoustic transmission between the drill bit and therock surface it is in contact with, but in this position there is nodirect metallic contact between the drill string and the drill bit.Piezo-electric elements 19 transmit a signal into the drill bit, thepiezo-electric elements acting against reaction mass 20. After a shorttransmission signal a return signal is received via the drill bit andpiezo-electric elements 21.

The problem with the arrangement illustrated in FIG. 1 is that it has towork in a very hostile environment in a borehole where various rubbercomponents, seals and moving joints are subjected to very abrasive mudflow, high temperatures and pressures, and also a chemically hostileenvironment. This is particularly a problem because failure of theapparatus may result in a considerable down time while the drill stringis removed and then replaced.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided adrill string sub assembly comprising a section of tube, a springconnected to the tube at a point remote from a first end of the sectionof tube, and a mass connected to the spring such that it can vibrate onthe spring in an axial direction relative to the tube, the value of themass and stiffness of the spring being selected such as to act as amechanical filter and damp longitudinal compression waves at apredetermined frequency or frequency band received at the first end ofthe section of tube.

A drill string sub assembly in accordance with the invention employing amass on a spring (which spring may be of any suitably resilientcoupling, for example an annular disk with a mass supported at its innerperiphery or may comprise the stiffness of a portion of the section oftube itself to which the mass is attached) enables the first portion ofa drill string subjected to longitudinal compression waves to bemechanically isolated from a second portion at particular frequencies.This is particularly advantageous because it provides a means ofpreventing transmission of longitudinal compression waves in a drillstring without the need to provide a mechanical break.

Preferably the spring is an annular member located coaxially with thetube and is secured at a radially outer edge to the tube and at aradially inner edge to the mass. This enables the mass to act uniformlyon the tube in a plane perpendicular to the axis of the tube. The sameadvantage may be achieved if the radially inner edge of the spring ismounted to the outer surface of the tube whereby the mass would then belocated externally of the tube.

Preferably the filter comprises at least two springs connected toaxially separated points of the tube with each spring connected to arespective mass. This is particularly advantageous if the springs are ofdifferent stiffnesses and/or the masses are of different values, forthen the respective spring/mass combinations will act to suppressdifferent frequencies and cause the filter to act as a band stop filterover a range of frequencies. Various mass spring combinations can bebuilt up to act in a manner analogous to electrical inductive/capacitivecomponents in a ladder network.

In accordance with a second aspect of the invention there is provided adrill string comprising borehole surveying apparatus and a drill stringsub assembly in accordance with the first aspect of the invention foracoustically isolating at least a portion of the surveying apparatusfrom the drill string. The section of tube of the filter enables theportion of surveying apparatus to be connected to the drill stringwhilst enabling the portion to be acoustically isolated at particularfrequencies from the drill string. This is particularly advantageous ifthat portion of the surveying apparatus comprises surveying equipmentwhich may either monitor for acoustic signals or transmit and/or receiveacoustic signals, and which therefore needs to be isolated from thedrill string to prevent either interference by signals received via thedrill string, or loss of a transmitted signal through the drill string.

According to a third aspect of the invention there is provided a drillstring comprising a drill bit; an acoustic transducer arranged tovibrate the drill bit such that acoustic signals are transmitted intorock ahead of the drill bit; and a drill string sub assembly inaccordance with the first aspect of the invention, arranged such as toisolate the remainder of the drill string from signals generated by theacoustic transducer. This is particularly advantageous, for as discussedin the introductory portion of this specification, there are certainapplications where it is desirable to transmit an acoustic signal via adrill bit and where it is necessary therefore to acoustically isolatethe drill bit from the remainder of the drill string. However, it isnecessary that any such isolation means also enables a very high torqueto be transmitted to the drill bit during a drilling operation and alsoensures very high compressional and tensile forces can be appliedbetween the drill string and drill bit.

Employing a drill string in accordance with the present inventionenables acoustic isolation between a drill bit and drill string to beachieved without having to employ the mechanical break disclosed in theapparatus illustrated in FIG. 1 and the inherent problems associatedwith the necessary seals and the like, which lead to reliabilityproblems in such a hostile environment.

Employing the present invention the drill bit is mechanically connectedto the drill string via the section of tube of the drill string subassembly such that mechanical forces are transmitted directly from thedrill string to the drill bit via the section of tube, the thickness ofthe wall of the section of tube having a low compliance relative to thewall of the drill string, which permits acoustic decoupling incombination with the mass/spring combinations. Preferably the mass, oreach mass, is annular and arranged around the inner surface of thesection of tube, the filter being arranged such that drilling mud canflow through its centre. In this way the filter effectively provides acontinuation of the drill string enabling the drilling mud to betransmitted to the drill bit.

The spring may have a very high stiffness depending on the value of theassociated mass and frequency of the transducer. In certain applicationsthe spring may comprise at least in part the section of tube itself, inthat the tube may flex about the point at which the mass is connected.

Preferably the centre of gravity of each mass is between the point atwhich the mass is connected to the spring and the drill bit. It isnecessary for there to be a certain compliance in the section of tubebetween its first end and the point at which it is connected to themass. Having the centre of gravity of the mass between its point ofconnection to the spring and the drill bit minimises the distance themass need extend the other side of the spring and therefore minimisesthe overall length of the mechanical filter.

Preferably the apparatus further comprises: an inner tube which in useisolates the mass from the drilling mud, which tube is sealed about themass to define a volume between the inner tube and the inner surface ofthe section of tube, in which volume the mass is located, this volumebeing filled with a fluid. The apparatus also comprises means formaintaining the fluid at a pressure substantially equal to the pressureon the outer surface of the section of tube. The outer surface of thetube is subjected to very high pressures, and thereby maintaining afluid which acts against the inner surface of the section of tube at apressure equal to the external pressure ensures that the tube is notdistorted.

One embodiment of the present invention will now be described, by way ofexample only, with reference to FIGS. 2, 3 and 4 of the accompanyingdrawings, of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a previous device schematically.

FIG. 2 illustrates schematically the principal components of themechanical filter employed in the borehole drilling apparatusillustrated in FIG. 3;

FIGS. 3(A,B) are longitudinal cross-sections through drilling apparatusin accordance with a third aspect of the present invention; and

FIG. 4 illustrates the modelled performance characteristics of theapparatus illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 2 there is schematically illustrated borehole drillingapparatus comprising a drill string sub assembly in accordance with afirst aspect of the present invention, the apparatus comprising a drillbit 20 connected to drill string 21 via drill string sub assembly whichacts as a mechanical filter, indicated generally as 22. Attached to thedrill bit is an acoustic transducer comprising ceramic stack 23 whichacts against reaction mass 24 to transmit acoustic signals into thedrill bit, which signals are then propagated into the rock (not shown)to be drilled. The filter 22 comprises a cylindrical tube 25 which has arelatively thin compliant wall relative to the thickness of the wall ofthe drill string 26. Annular masses 27 and 28 are connected to tube 25of the filter via springs 29, 30 which are in the form of annularwashers, the outer periphery of which is welded to the tube 25 with theinner periphery welded to the associated mass. A layer of compliantmaterial 31 extends between the tube and the masses in order to ensurethat excessive loads are not placed on the springs 29, 30 in transit orduring abnormal conditions, but the compliance of the material is suchas to have a negligible effect on the resonant properties of thespring/mass pairs 29, 27 and 28, 30.

The stiffness of the springs 29, 30 and the value of their respectivemasses is selected such that they define a blocking frequency within thefrequency range at which the transducer 23 is operated. The blockingangular frequency ω_(n) of the nth mass spring combination is determinedas

    ω.sub.n =√K.sub.n /M.sub.B,n                  (1)

where

K_(n) is the nth spring constant,

M_(B),n is the nth blocking mass.

The two masses and/or spring constants are preferably of differentvalues such that a blocking function is achieved over a broader range offrequencies. The position of the masses is not critical but the masseswill be more effective the more compliance there is in the section ofthe tube of the filter between the point at which the mass is connectedand the drill bit. Similarly there needs to be sufficient compliance inthe section of tube between the two springs to avoid interaction betweenthe two masses.

Referring now to FIGS. 3a and 3b, there is shown respectively incross-section the lower part and upper part of drilling apparatusemploying a practical implementation of the filter illustratedschematically in FIG. 2. Referring to FIG. 3a, drill bit 30 is connectedby tapered screw thread 31 to adaptor 32 which is in turn connected byscrew thread 33 to a first compliance section 34. First compliancesection 34 comprises a thin cylindrical tube of a material having a highstrength:mass ratio and a relatively low Young's Modulus, such astitanium. Around the lower portion of the first compliance section is ametallic band termed the near bit stabiliser which follows the wall ofthe borehole, indicated by broken line 36, and acts to stabilise thedrill bit during drilling. The first compliance section 1 extendsvertically, as shown in the drawing, to first annular spring 37 to whichit is welded at 38. To the other side of the annular spring 37 is weldedat 39 a second compliance section 40 having the same properties as thefirst compliance section.

To the inner surface of the first annular spring is attached a firstannular mass 41 comprising portions 41a and 41b joined together byrespective threaded portions 42 such as to sandwich the first annularspring and thereby retain the mass 41 in place. Between the mass 41 andfirst and second compliance members 34 and 40 there is a layer ofcompliant material 43 such as rubber which serves to prevent the mass 41"rattling" within the compliance sections.

Within the passage defined by the annular mass 41 is a mud flow pipe 44which as can be seen from FIG. 3b extends from the drill string to theadaptor 32 and conveys mud to the drill bit 30.

Located below the mass 41 is reaction mass 45 comprising a densematerial, for example tungsten. The reaction mass 45 comprises two parts45a and 45b between which are sandwiched straps 47. The two parts of thereaction mass are bolted together by bolts 46.

The bias straps 47 extend to the far end of piezo-electric transducer 48and maintain piezo-electric elements comprising the piezo-electric stackin compression. The piezo-electric transducer 48 comprising transmitterelements 48a and receiving elements 48b. The transducer is securelyaffixed to adaptor 32 by bolts 49, and therefore in turn to drill bit30.

Referring now to FIG. 3b, the second compliance member 40 is seen toextend to second annular spring 50 to which it is welded, the springalso being welded to third annular compliance member 51. It should benoted that the second annular spring 50 is less substantial than thefirst annular spring and therefore has a lower stiffness constant.Spring 50 supports annular mass 52 comprising portions 52a and 52b inthe same manner as first annular mass 41 is supported by first annularspring 37. The mud flow pipe 44 is sealed at its upper end with theupper portion of the third compliance section 51 via annular piston 53.The volume 54 formed between the three respective compliance sectionsand the mud flow pipe, in which volumes the annular masses are located,is filled with oil which is maintained at a pressure equal to that ofthe mud in the borehole external to the compliance sections, thispressure being exerted by mud entering through aperture 55 exerting aforce on piston 53. Bulkhead 56 seals the mud flow pipe 44 to the thirdcompliance section 51.

The top of the third compliance section is threaded to connector ring 57which in turn is threaded to insert ring 58. This can be considered asthe start of the drill string. Insert ring 58 comprises electronicmodules 59 for processing and transmitting signals received from theacoustic transducer via wires 59 which run via slots in the drill pipe44 to the transducer 48. The insert ring 58 connects to the bottom ofthe steel drill pipe which forms the rest of the drill string.

Drilling would be paused while the transducer is in operation and theweight on the drill bit varied in the same manner as disclosed in theearlier UK patent application.

The material of the annular masses depends on the relative dimensionsand the required mass and could for example be steel. The same appliesfor the first and second annular rings where the material used woulddepend on the stiffness required but it may be appropriate to use thesame material as the compliance sections to which they are welded.

As previously stated the exact value of the various components willdepend upon the application but could be derived by an appropriatelyskilled engineer in this field. However, typical values are given belowfor a standard 8.5" drill bit attached via a filter to a standard 8.75"drill collar, the filter being designed to reject the -30 dB level overat least 1,000 to 2,000 Hz frequency band. Typical characteristics of asuitable two stage filter are as follows.

Terminating collar material: Steel

Collar density 7800 kg/m3

Collar wave speed 5190 m/s

Thinwall Material: Titanium

Thinwall density 4510 kg/m3

Thinwall wave speed 4614 m/s

Common outer radius for filter section and collar wall 0.09 m.

Thinwall thickness 10 mm

Collar wall thickness 31.5 mm

Annular Mass Material: Steel

Density of mass metal 7800 kg/m3

Wavespeed in mass metal 5190 m/s

Outer radius in mass metal 0.077 m

Inner radius in mass metal 0.046 m. Mass/unit length therefore=93.44kg/m

Number of stages in filter 2.

Length of compliance sections:

First=0.914 m

Second=0.758 m

Third=0.354 m

Mass lengths 0.807 m (first) and 0.689 m (second)

Blocking masses

First=75.4 kg

Second=64.3 kg

Blocking resonant frequencies

First=2087 Hz

Second=794 Hz

Basic blocking frequency formula is f=(K/M)^(1/2) /(2π), where K=springconstant, and M=blocking mass. NB for very long masses, considerationhas to be given to standing wave effects in the masses to get a precisevalue for the spring constant, the determination of which thus becomesmore complicated.

In the present example, the blocking spring constants, K_(n) are thusapproximately

    K.sub.1 =1.3.10.sup.10 N/m

and

    K.sub.2 =1.6.10.sup.10 N/m

FIG. 4 illustrates the modelled fractions of total power emerging fromthe transducer which go respectively into the rock and into the drillcollar, expressed in decibel terms, and plotted as functions offrequency. The band-stopping action of this two stage filter is evidentbetween about 800 Hz and 2200 Hz. The two individual blockingfrequencies are visible as minima in the lower curve.

The present invention has been described with reference to anapplication of drilling apparatus. However the mechanical filter aspectof the invention may be used in other applications within the scope ofthe claims appended hereto.

What is claimed is:
 1. A drill string assembly comprising:a section oftube, with a first end, wherein the first end directly, mechanicallylinks a drill pipe to a drill bit and transmits rotational forces fromthe drill pipe to the drill bit during drilling; a spring connected tothe section of tube at a point remote from the first end of the sectionof tube; and a mass connected to the spring such that the mass canvibrate on the spring in an axial direction relative to the section oftube and drill string, the value of the mass and the stiffness of thespring being selected such as to cause them to act as a mechanicalfilter and damp longitudinal compression waves at a predeterminedfrequency of frequency band and received at the first end of the sectionof tube.
 2. A drill string sub assembly as claimed in claim 1 whereinthe spring is an annular member located coaxially with the tube and issecured at a radially outer edge to the tube and at a radially inneredge to the mass.
 3. A drill string sub assembly as claimed in claim 1comprising at least two springs connected to axially separated points ofthe tube with each spring connected to a respective mass.
 4. A drillstring sub assembly as claimed in claim 3 wherein the springs are ofdifferent stiffnesses.
 5. A drill string sub assembly as claimed inclaim 3 wherein the masses are of different values.
 6. A drill stringcomprising:borehole surveying apparatus; and a drill string sub assemblyas claimed in claim 1, the drill string sub assembly acousticallyisolating at least a portion of the borehole surveying apparatus fromthe remainder of the drill string.
 7. A drill string as claimed in claim6 comprising an acoustic transducer, wherein the drill string subassembly is tuned such that it acts as a band stop filter in thefrequency range of the acoustic transducer.
 8. A drill stringcomprising: a drill bit; an acoustic transducer arranged to vibrate thedrill bit such that acoustic signals are transmitted into rock ahead ofthe drill bit; and a drill string sub assembly as claimed in claim 1arranged such that it acts to isolate the remainder of the drill stringfrom signals generated by the acoustic transducer.
 9. A drill string asclaimed in claim 8 wherein the drill string sub assembly is tuned suchthat it acts as a band stop filter in the frequency range of theacoustic transducer.
 10. A drill string as claimed in claim 8 whereinthe drill bit is mechanically connected to the remainder of the drillstring via the section of tube of the drill string sub assembly suchthat in use mechanical forces are transmitted to the drill bit via thesection of tube, the thickness of the wall of the section of tube havinga low compliance relative to the wall of the remainder of the drillstring.
 11. A drill string as claimed in claim 9 or 10 wherein the massis annular and arranged around the inner surface of the section of tube,the drill string sub assembly being arranged such that drilling mud canflow through its centre.
 12. A drill string as claimed in claim 11wherein the centre of gravity of the mass is between the point at whichthe mass is connected to the spring and the drill bit.
 13. A drillstring as claimed in claim 11 further comprising: an inner tube whichisolates the mass from the mud and which is sealed about the mass todefine a volume between the inner tube and the inner surface of thesection of tube, in which volume the mass is located, this volume beingfilled with a fluid; and means for maintaining the fluid at a pressuresubstantially equal to the pressure on the outer surface of the sectionof tube.